A siamese cylinder block with small gaps (bore pitches) between cylinders is used in multi-cylinder engines of vehicle in the related art to reduce the size, weight, and entire length. The siamese cylinder bock is manufactured by casting aluminum with several cylinder liners, in which the pitches of the cylinder bores are formed short by removing water jackets (cooling water channels) between the cylinder liners.
In the siamese cylinder block having the above structure, the gaps between the cylinder bores (referred to as ‘siamese portion’) are small and it is difficult to form cooling water channels, such that it is vulnerable to heat. In particular, the siamese portion around the upper end of the cylinder block where heat is directly transferred from the combustion chamber is heated at considerably high temperature.
When the siamese portion is heated at high temperature, temperature difference from the surroundings occurs and the upper portion of the cylinder block is excessively deformed by heat, which deteriorates the degree of perfect circle of the cylinder bore. Further, the engine oil flows inside the combustion chamber through the gap between the piston ring and the inner wall of the cylinder block, such that the engine oil is wasted and blow-by gas increases. Accordingly, cooling the upper portion of the cylinder block and the siamese portion is very important to ensure sealability against the combustion gas, prevent the bores from being excessively deformed, and reduce the temperature of the pistons.
Researches for appropriately cooling the siamese portion by forming silts or drill holes, which function as cooling water channels, between the cylinder bores or forming a wedge-shaped water hole through the cylinder head gasket have been conducted in the related art; however, the cooling effect was insufficient.